Messages in this thread | | | Date | Fri, 27 Apr 2007 00:47:49 +1000 | From | Nick Piggin <> | Subject | Re: [00/17] Large Blocksize Support V3 |
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Mel Gorman wrote: > On (25/04/07 23:46), Christoph Lameter didst pronounce: > >>On Thu, 26 Apr 2007, Nick Piggin wrote: >> >> >>>Yeah. IMO anti-fragmentation and defragmentation is the hack, and we >>>should stay away from higher order allocations whenever possible. >> >>Right and we need to create series of other approaches that we then label >>"non-hack" to replace it. >> > > > To date, there hasn't been a credible alternative to dealing with > fragmentation. Breaking the 1:1 virtual:physical mapping and defragmenting > would incur a serious performance hit.
Depends what you mean by "dealing with", I guess.
I would say there has been no credible alternative to virtually mapping the kernel.
>>>Hardware is built to handle many small pages efficintly, and I don't >>>understand how it could be an SGI-only issue. Sure, you may have an >>>order of magnitude or more memory than anyone else, but even my lowly >>>desktop _already_ has orders of magnitude more pages than it has TLB >>>entries or cache -- if a workload is cache-nice for me, it probably >>>will be on a 1TB machine as well, and if it is bad for the 1TB machine, >>>it is also bad on mine. >> >>There have been numbers of people that have argued the same point. Just >>because we have developed a way of thinking to defend our traditional 4k >>values does not make them right. >> >> >>>If this is instead an issue of io path or reclaim efficiency, then it >>>would be really nice to see numbers... but I don't think making these >>>fundamental paths more complex and slower is a nice way to fix it >>>(larger PAGE_SIZE would be, though). >> >>The code paths can stay the same. You can switch CONFIG_LARGE pages off >>if you do not want it and it is as it was. >> > > > It may not even need that that much effort. The most stressful use of the > high order allocation paths here require the creation of a filesystem and > is a deliberate action by the user.
Saying "oh this stuff may not always work quite right for everyone, but it is OK because it is a special purpose solution for now" IMO is a big sign saying that it is a bad design, and including it means we're lumped with it forever.
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